In recent years, there has been an increased demand for improvement in the fuel consumption rate (fuel efficiency) of a hydraulic excavator and other hydraulic working machines.
A hydraulic excavator disclosed, for instance, in JP,A2003-329012 is configured so that a changeover valve is installed in a hydraulic line (fluid return hydraulic line) in which a returning fluid is distributed when a boom connected to a bottom side hydraulic chamber of a boom cylinder (hydraulic cylinder) is lowered and that a regeneration circuit having a hydraulic motor and a generator coupled to the hydraulic motor is connected to a downstream end of the changeover valve. The hydraulic excavator is capable of changing the position of the changeover valve in accordance with an operation mode selected on a monitor panel. More specifically, the hydraulic excavator can selectively allow the regeneration circuit to communicate with the bottom side hydraulic chamber in accordance with the selected operation mode (e.g., the regeneration circuit communicates with the bottom side hydraulic chamber during an excavation, whereas the regeneration circuit does not communicate with the bottom side hydraulic chamber during a micromanipulation). If, for instance, the selected operation mode causes the regeneration circuit to communicate with the bottom side hydraulic chamber when the boom is lowered, the returning fluid discharged from the bottom side hydraulic chamber when the boom is lowered drives the hydraulic motor and the generator to generate a regenerative current. However, the hydraulic excavator does not achieve regeneration unless an appropriate operation mode is selected by an operator. Due to a complicated operation mode changeover procedure, regeneration might not readily be achieved when it is required.
A hydraulic excavator disclosed, for instance, in JP,A2007-107616 is developed in view of the above circumstances. This hydraulic excavator includes a branching unit, a regeneration circuit, and a flow regulating circuit. The branching unit branches a fluid return hydraulic line into two or more hydraulic lines. The regeneration circuit operates so that part of hydraulic fluid branched by the branching unit is introduced into a tank through a hydraulic motor connected to a generator. The flow regulating circuit operates so that the remaining hydraulic fluid branched by the branching unit is introduced into a tank through an orifice (flow regulating means), which changes a passage flow rate in accordance with the operation amount of a control lever. In other words, this hydraulic excavator aims at providing a proper amount of regenerated energy without causing an abrupt change in operability by controlling the flow rate of returning fluid introduced into the regeneration circuit and into the flow regulating circuit in accordance with the amount of control lever operation for boom lowering.